The overall aim is to determine how visual and kinesthetic sensory information is used online to guide movements. A better understanding of these processes is important for rehabilitation and the development of orthotic devices for patients with senson-motor deficits. There are two principle aims. One major aim is to provide a foundation for understanding the neural processes in tactile handling - the manipulation of tools. To this end, we will study the motions and forces developed by the fingers in grasping and manipulating objects, and the processes whereby kinesthetic information can be used to identify an object by its shape. The second major aim tests the hypothesis that gaze signals, of extraretinal origin, provide a major input to the limb motor control system. Visual guidance of grasping movements will be studied by comparing the kinematics of the hand and fingers when visual and or tactile information is unavailable with the behavior when this information is present. The temporal evolution of hand and finger kinematics will be characterized, as will be the times at which various sources of information become important. The guiding hypothesis of this study is that there exist a few temporal patterns of coordination (synergies) that account for most of the variations in finger kinematics. The control of grip forces in a tripod grasp will be studied under non-equilibrium conditions - for example, when the grasped object is rotated. Previous investigations restricted to static, equilibrium conditions had uncovered a simple synergy and the proposed investigations will determine how this synergy is modified under more general experimental conditions. The third part of the first aim will be to determine the extent to which kinesthetic information can be utilized to deduce hand trajectories, in particular, distortions in the kinesthetically derived information about curvature and rate of change of curvature of hand trajectories will be determined. The ultimate aim of this project is to define the algorithms whereby information about endpoint trajectories are derived from biological variables such as muscle lengths or joint angles. The hypothesis underlying the second aim is based on previous observations that errors in gaze induced by visual illusions are accompanied by errors in limb motor control in pointing or interception tasks. To test this hypothesis, ocular and manual tracking performance will be investigated in patients with spinocerebellar ataxia. Since the cerebellum is intricately involved in tracking, this task is a very sensitive test of cerebellar function and has been found to elicit errors in this patient population. The hypothesis predicts that errors in ocular and manual tracking should be reliably correlated in time and in amplitude.